(1) Field of the Invention
This invention relates to a magnetic resonance imaging method and apparatus utilizing NMR (nuclear magnetic resonance), and more particularly to a magnetic resonance imaging method and apparatus for obtaining images by the fast spin echo method.
(2) Description of the Related Art
Conventional magnetic resonance imaging methods include a fast spin echo method using pulse trains called RARE (Rapid Acquisition with Relaxation Enhancement). In the fast spin echo method based on RARE, a single excitation RF (Radio Frequency) pulse (also called a 90.degree. pulse since the spin phases of protons are rotated 90.degree.) and then a plurality of refocusing RF pulses (also called 180.degree. pulses since the spin phases of protons are rotated 180.degree.) are emitted to an examinee to generate echo signals (primary echoes) successively. These echo signals are subjected to different phase encoding to obtain data of a plurality of lines in a raw data space at a time. In this way, the excitation RF pulse emission is repeated a reduced number of times to expedite image pickup.
A frequency profile produced by the refocusing RF pulses, ideally, is in a square wave. In practice, however, this is an imperfect square wave having rounded edges. Because of this imperfection, there always occur some magnetization vectors having tilt angles only at 90.degree., and not 180.degree., due to the spins of protons in the examinee's body. As a result, spurious echo signals are inevitable. In the conventional multiecho method, these spurious echo signals cause various artifacts such as a mirror artifact with a mirror-reversed image due to the spurious echo signals superimposed on a normal image due to proper echo signals. A proposal has been made heretofore to control the phases of RF pulses for canceling the spurious echo signals, thereby to eliminate the influences of the spurious echo signals (Y. Zur et al; Journal of Magnetic Resonance 71, 212-228, 1987).
However, spurious echo signals contain information for visualizing images. Image contrast could be enhanced by utilizing such information. Canceling the spurious echo signals as in the prior art results in poor signal utilization efficiency.
In the RARE method, a CPMG pulse train (Carr-Purcell-Gill-Meiboom Impulse Folge) is formed. When this pulse train occurs, the period and strength of application to gradient fields are adjusted so that rephasing conditions are satisfied for all the gradient fields. Thus, the spurious echo signals and proper echo signals are brought into phase, whereby the spurious echo signals as well as the proper echo signals are utilized to visualize images (Japanese Patent Publication (Unexamined) No. 61-77748).
However, a gradient field coil disposed inside a main magnet which forms a static magnetic field, owing to eddy currents or the like caused by leakage from the gradient fields, disturbs the static field which is otherwise maintained uniform by the main magnet. This gives rise to the problem of obstructing spurious echo signals and proper echo signals coming into phase. In order to solve this problem, a special, shielded gradient field coil is required for preventing leakage of the fields to avoid disturbance of the static field formed by the main magnet.